Last year was widely perceived to be a year of outrageous e-book growth, but some new research suggests otherwise. According to new data from Bowker and the Book Industry Study Group, the number of book buyers who also purchased an e-book increased by 17 percent in 2011, compared to 9 percent in 2010 – well below the 25 to 30 percent growth that some had hoped for.

To be sure, this growth varied by genre, noted Bowker’s Kelly Gallagher at Digital Book World this morning. (Bowker conducted this research with the Book Industry Study Group.) E-books now make up 26 percent of adult fiction purchases, compared to 11 percent of children’s book purchases and 3 percent of cookbook purchases.

Seventy-four percent of book buyers have never bought an e-book (and 14 percent of those actually own an e-reader or tablet but choose not to use it to read e-books).

Bowker looked at the habits of “power buyers” — people who purchase four or more print or e-books per month. “Whether print or digital, publishers’ best customers now look alike,” said Gallagher.

Print power buyers make up 22 percent of the overall print book-buying population, and they drive 53 percent of print book purchases overall.

Meanwhile, e-book power buyers make up 35 percent of the overall e-book buying population, but they drive 60 percent of overall e-book purchases. In other words, about a third of the overall buyers drive two-thirds of overall purchases. Casual e-book buyers “are not pulling their weight” compared with casual print book buyers, Gallagher said.

There’s a bright spot for e-book growth: Around 7 to 12 months after buying their first e-book, 72 percent of power buyers switch over to e-books exclusively.

Overall, though, print power buyers slowed their entry into the e-book marketplace in 2011. “That’s why we didn’t see exponential growth,” Gallagher said. And beyond those power-buying voracious readers, casual e-reading is at risk: As more people buy tablets, e-reading becomes just one option among many.

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